bug#35543: Installer crashes on retry when selecting keyboard layout

[pelzflorian (Florian Pelz)]

If the first install fails (e.g. when the build fails, see separate
bug report on MATE install), it is possible to retry the installation.
However, on the second attempt the installer crashes when selecting a
keyboard layout.

A backtrace is displayed; the important part appears to be this:

In ./gnu/installer/steps.scm:
   177:21 10 (_)
In ice-9/eval.scm:
    619:8  9 (_ #(#(#(#<directory (guile-user) 2525140>) #<<ins…
In ./gnu/installer/keymap.scm:
    163:7  8 (_ _)
In unknown file:
           7 (scm-error misc-error #f "~A" ("Unable to locate ke
In ice-9/boot-9.scm:
   751:25  6 (dispatch-exception 4 misc-error (#f "~A" ("Unable
In ice-9/eval.scm
    619:8  5 (_ #(#(#<directory (guile-user) 2525140> #<<instal…
[some not important stuff I am too lazy to type…]
ice-9/eval.scm:159:9: Unable to locate keymap update file